Recent years in Taiwan area, the development of economy and the growth of national income lead to a rapid increase in the number of automobiles, since automobiles can be utilized for the purposes of commercial, industrial and leisure activities. The huge number of automobiles causes heavy traffic jams. Motorcycle is more moveable and easier to park, so it plays an important role in the medium- and short-distance transportation systems. In Taiwan area, the number of motorcycles with two-stroke engines is about 70% of the total number of motorcycles. A motorcycle with two-stroke engine is advantageous in that it has a simple engine structure, light weight, small volume, high engine output and easy maintenance. On the other hand, however, motorcycles with two-stroke engines cause the problem of serious air pollution due to their inlet and outlet system design. The result of investigation on contamination sources in Taiwan area reveals that 29% of the total carbon monoxide (CO) discharge and 20% of the total hydrocarbons discharge come from motorcycles with two-stroke engines. Therefore, in order to help solving the traffic jam problem, it is imperative to overcome the air pollution caused by motorcycles with two-stroke engines. For the purpose of efficient control of the air pollution by motorcycles, the Environmental Protection Administration in Taiwan promulgated the Second Stage Motorcycle Emmision Standard on 1991, which defines the upper limit of the total amount of hydrocarbons and NO.sub.x as 3.0 g/km and the upper limit of CO amount as 4.5 g/km. In order to meet the requirements in the abovesaid regulation, several big motorcycle manufacturers have aggressively made efforts to research and improve engine combustion and exhaust systems of motorcycles. Since July of 1992, all the two-stroke motorcycles from the OEM have to be equipped with catalytic converter to comply with the tight regulation.
In addition to the requirements in the respect of environmental protection, motorcycle manufacturers also keep on doing research and develop to enhance the engine performances of two-stroke motorcycles, thereby pushing lubricating oil manufacturers to provide products more fit to the updated requirements. A result directly caused by the requirements is that lubricating oils with better capability of protecting high-performance engines are needed.
The lubricating system for two-stroke engine is supplied by the mixture of two-stroke oil and fuel. Mixed gas passed from the intake port into the combustion chamber. The passing oils stick to the gas cylinder and piston is thus providing the necessary lubricity for the two-stroke engine. In order to prevent the deposit formed in the combustion chamber and piston, there is no traditional ash type additives (i.e., ZDDP) contained in the two-stroke oil formulation, so the lubricity is mainly contributed from the oil film of the base oil. A base oil may contain mineral and/or synthetic oils.
Adding a high molecular weight polyisobutylene (PIB) into an engine oil can enhance the oil's lubricity, but the carbon deposit may be increased and thus the oil's detergency is decreased. Since in principle detergency conflicts with lubricity, how to make a balance between them in order to obtain an optimum effect is still a problem not solved in the industry.